Hopes for a quick recovery faded as inspections revealed extensive damage to the tank's insulation foam.

Nasa has been particularly sensitive about shuttle tank safety since losing Columbia and seven astronauts in 2003 as a result of a debris strike from tank foam insulation.

The impact, which occurred during lift-off, damaged the shuttle's heat shield, which failed as Columbia soared through the atmosphere.

With temperatures reaching as hot as the surface of the Sun, the ship broke apart, dooming the astronauts.

Atlantis' flight has been rescheduled for no earlier than 8 June, effectively bumping a fifth mission from this year's flight roster and jeopardizing the Columbus module launch on what would be the fourth and final flight of the year.

The module is at the Kennedy Space Center in Florida where it is being prepared for launch.

"Flying four flights is not outside the realm of possibility, but we want to do this in a safe and orderly manner and we'll just see how it works out," said shuttle programme manager Wayne Hale.

Work to repair the shuttle's fuel tank is under way in Florida. Engineers have identified 2,664 dings, gouges and other damage sites that need to be fixed before the shuttle can fly, said John Honeycutt, an external tank program manager.

Some areas are so heavily pitted that technicians will apply a new layer of foam rather than attempt individual repairs, he added.

Nasa must also prove that the repaired areas will be safe to fly.

Bill Gerstenmaier, the associate administrator for spaceflight, said he expects the shuttle programme to digest the delays within a year or so and that there will still be enough time to complete the space station assembly before the shuttles are retired in 2010.